1. Field of the Invention
The present invention relates to a lithographic apparatus, a control system, and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The control system of the known lithographic apparatus controls and operates directly on conventional coordinates such as determined positions (note that in this application the term position can include the orientation of a moveable object of the lithographic apparatus [so one position can include six degrees of freedom]). Furthermore, it is noted that the control system can include a plurality of sub-control systems, wherein each sub-control system is configured to control a position of one of the moveable objects. Examples of relevant moveable objects of the lithographic apparatus are: lens elements, the substrate table with the substrate, the reticle stage with the reticle, etc.
Currently, desired imaging characteristics like focus, magnification, distortion, etc. expressed in native coordinates are translated into conventional setpoints in conventional coordinates (positional/spatial coordinates) for the relevant moveable objects of the lithographic apparatus by means of the inverse of a known sensitivity matrix. Next, the thus determined conventional setpoints are compared with components of the information signal (also expressed in conventional coordinates) which yields conventional error signals to be fed to a conventional controller. Generally, each conventional error signal is fed to a corresponding and unique conventional controller for generating a control signal (in a conventional coordinate) for controlling the corresponding element of the lithographic apparatus. This means that a position error in one of the elements can be solved only by the unique (local) controller for this object of the lithographic apparatus.
In the conventional control system the relation between the position of the lens element and the stages, and the imaging characteristics, is known. In other words, if the positions of these objects is known, the effect on focus, magnification, etc. can be calculated. Now, if a change in the imaging characteristics is required, a desired position for each individual object can be calculated. These desired positions function as a setpoint for the various unique and local control loops, which means that each of the objects to be positioned has its own control loop. This conventional scheme has some drawbacks.
A first drawback is that if one of the moveable objects gets disturbed, the control loop of this object has to correct the situation all by itself. For example, if one of the lens elements gets disturbed but happens to have a low servo bandwidth, it takes some time before the imaging characteristics are back to their optimal value.
A second drawback is that if a setpoint change occurs for one of the imaging characteristics (e.g., focus), this change results in a setpoint change for all elements that have an effect on focus. Because not all elements react with the same speed, disturbances on other imaging characteristics that do not have a setpoint change occur.